25: composition of the earth

p-waves

seismic waves that move through liquids and solids

s-waves

seismic waves that do not move through liquids.

seismic wave behavior

speed changes with rock density; soft layers absorb waves; waves refract at density differences.

lithosphere

strong, brittle uppermost layer.

asthenosphere

plastically deformable mantle layer.

mantle

includes upper and lower mantle.

core

liquid outer core, solid inner core.

magnetic field source

moving liquid rock in outer core, produces north and south poles

geomagnetic reversals

common polarity flips in earth’s history.

crust elements

oxygen abundant, 46.6%

silicon abundant 27.7%

crust contains lighter elements.

crust vs mantle

crust - aluminum and silicon

mantle - iron and magnesium

igneous rock formation

solidifies and crystallizes from magma or lava.

intrusive (plutonic)

cools within earth.

extrusive (volcanic)

forms on the surface.

mafic

high in iron/magnesium; dark and dense. (gabbro)

felsic

high in silica; light and less dense. (granite)

igneous combinations

any combo of intrusive/extrusive with mafic/felsic.

pluton

igneous body cooled below surface; exposed over time.

batholith / laccolith / volcanic neck / sills / dikes

igneous features that create iconic landscapes.

sedimentary processes

transport in high energy,

deposition in low energy

compaction and cementation.

clastic

sediment compacted and cemented by overlying weight.

chemical

precipitation of dissolved chemicals.

evaporites

crystallized salt deposits from evaporating water.

examples of sedimentary rocks

sandstone, shale, conglomoerate

white cliffs of dover

composed of calcium carbonate.

metamorphism causes

heating, pressure, both, or compression/shear.

contact metamorphism

magma heats adjacent rock, changing minerals.

regional metamorphism

pressure from overlying rock alters minerals.

foliated

aligned crystals due to unequal pressure and shearing.

nonfoliated

no alignment; forms without shear; recrystallization increases density.

surface rocks on earth

sedimentary is the most common surface rock on earth

earth crust composition

96% igneous crust

75% exposed rocks are sedimentary.

rock cycle

rocks are continually created, eroded, modified.

operates at the surface, operates at the subsurface

surface = hydrologic cycle

subsurface = tectonic cycle

wegener proposal

continents move; proposed in 1912.

evidence: matching coastlines

continents fit like puzzle pieces.

evidence: fossils

similar fossils found on distant continents.

similar plants on 3

freshwater reptiles on 2

small reptiles across many

evidence: rocks

similar rocks on distant continents

appalachian and caledonian mountains share geology.

evidence: glacial deposits

similar flow patterns across continents.

evidence: species

related species appear on multiple continents.

seafloor spreading process

magma rises through cracks in thin ocean crust, cools, forms new ocean floor with ridge shape.

seafloor density contrast

continental crust ~2.7 g/cm³ - less dense

oceanic ~3.0 g/cm³ - more dense

denser oceanic crust subducts.

earthquake evidence

quakes common along ridges and trenches.

paleomagnetism

rocks record magnetic field orientation as they cool;

reversals preserved symmetrically around ridges.

age of crust

youngest crust at ridges

oldest near subduction zones.

lithosphere plates

earth’s lithosphere is divided into moving plates.

isostasy

continents float flexibly on denser mantle.

slab-pull model

convection drives motion; slab pull at subduction zones; ridge push from new crust.

divergent boundary

plates move apart.

divergent oceanic-oceanic

forms midocean ridges.

divergent continental

forms rift valleys.

east african rift

active divergent rift; forming a future ocean basin.

keweenawan rift

ancient failed divergent rift in north america; formed lake superior.

convergent boundary

plates collide.

convergent oceanic-oceanic

denser plate subducts; trench + island arcs form

convergent oceanic-continental

oceanic plate subducts; volcanic arc form + metamorphism.

convergent continental-continental

no subduction; mountains build.

himalayan uplift

convergent: india collided with eurasia; formed himalayas; affected global climate.

tonga volcanic arc

convergent: collision of australian-indian and pacific plates; wadati-benioff zone present.

flat subduction (laramide orogeny)

shallow subduction forming sierra nevada and rockies.

transform boundary

plates slide past each other; shallow earthquakes.

san andreas fault

transform: pacific vs north american plate; loma prieta 1989; san francisco 1906.